Another eBay deal came my way 
This time in the shape of some Polar Pro ND filters.
I thought I’d share some details of the fitment of X3 filters on the Z3 camera, as there are noticeable differences.
First up, some side by side comparisons.
Pay close attention to the bezels on each, which holds a clear removable glass lens ‘cover’. Z3 on the left, X3 on the right.
Z3 with the bezel removed, ready to screw on the ND filter:
X3, bezel removed:
The Z3, with a Polar Pro filter attached:
While it screws on tight and with no issues, there is a very small gap around the outside, between the filter and the camera body:
The gap is more noticeable from the front. However it doesn’t let anything in, as in, no dust or dirt is able to get inside the camera.
And for reference, with the original bezel back in place:
So the answer is yes, the X3 filters do also fit the Z3 camera 
A minor mod was also required to the Z3 case, in order to leave a filter on, just a few mm needed to be shaved off the foam to allow for the deeper Polar Pro filter.
Sorted